1208
Energy & Fuels 1994,8, 1208-1216
Effect of the Occurrence and Modes of Incorporation of Alkalis, Alkaline Earth Elements, and Sulfur on Ash Formation in Pilot-Scale Combustion of Beulah Pulverized Coal and Coal-Water Slurry Fuel Sharon Falcone Miller? and Harold H. Schobert* Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received September 13, 1993. Revised Manuscript Received July 19, 1994@
Beulah (North Dakota) lignite was fired in both pulverized and coal-water slurry form to study the effect of fuel form and rank on ash formation processes. The fuels were burned in a downdired pilot-scale combustor at 316 M J h . In both of the Beulah fuels, organically bound sodium was important in the formation of micrometer and submicrometer sodium sulfate particles and coatings on larger silicate and aluminosilicate particles. The presence of sulfates indicates the importance of sulfur fixation by alkaline elements during combustion. Organically bound calcium is highly reactive within the char particle, participating in the formation of mixed aluminosilicates during char burnout; calcium is not associated with sulfur and does not appear to react outside of the char particle. The form in which the Beulah coal was fired, i.e., pulverized or as a CWSF, had no appreciable effect on the behavior of the alkalis and sulfur during combustion.
Introduction Many of the inorganic species in coals can be incorporated in the coal in various ways, including as ionexchangeable cations, as coordination complexes, and as discrete mineral phases1 The way in which an element will react during ash formation depends on the way in which it is held in the coal. The fact that a given element often occurs in more than one of these modes in the coal, and hence can exhibit several types of behavior, adds to the complexity and number of possible inorganic reactions that may occur during combustion. Therefore, it is important to know the distribution of the inorganics in coal t o understand their role in the formation of ash. The elements which occur in coal as organically bound cations include alkali metals and the alkaline earth elements. Cations of these elements are associated with carboxyl groups in the coal structure.2 The cations are usually well dispersed throughout the coal and are extremely reactive during the early stages of combustion, being released via decarboxylation of the coal structure at temperatures much lower than that at which elements incorporated in minerals will become available for reaction. Generally, carboxyl groups are not important components of the structure for coals of greater than about 80% carbon3 Thus the amount of alkali of alkaline earth cations will not be appreciable for coals of rank greater than subbituminous A or highvolatile C bituminous. Present address: Energy and Fuels Research Center, The Pennsylvania State University, University Park, PA 16802. Abstract published in Advance ACS Abstracts, September 1,1994. (1)Falcone, S.K.; Schobert, H. H. In Mineral Matter and Ash in Coal;Vorres, K. S., Ed.; American Chemical Society: Washington, DC, 1986; Chapter 9. (2) Huffman, G. P.; Huggins, F. E. In The Chemistry ofLow-Rank Coals; Schobert, H. H., Ed.; American Chemical Society: Washington, DC, 1984; Chapter 10. (3)van Krevelen, D. W. Coal: Typology-Chemistry-Physics-Constitution; Elsevier: Amsterdam, 1981; Chapter E. +
@
0887-0624/94/2508-1208$04.50/0
Elements which occur as organically bound cations are much more volatile than mineral-bound elements. Organically bound alkalis and alkaline earth elements are readily volatilized and maintained in the gas phase at typical flame temperature^.^ In addition, the volatility of organically bound elements is enhanced by the locally reducing conditions they experience during decarboxylation. The fraction of alkalis and alkaline earths that is released as vapor may vary from nearly 0 to 100%(ref 5 ) . As the cations are released, they may react with gaseous sulfur species to form sulfates, react with other inorganic volatiles to form new inorganic species, oxidize, or react with the existing mineral matter in the char.6-8 Alkalis and alkaline earth elements that volatilize in the flame later condense in cooler zones beyond the flame zone. However, sulfation of alkalis and alkaline earth elements and reactions with flame-borne silicates are important in forming submicrometer particles in the hot combustion zoneeg For example, sodium is vaporized in the form of sodium metal or cations, depending on the local reducing or oxidizing a t m ~ s p h e r e .Organically ~ bound sodium vaporizes below typical peak temperatures of pulverized coal flames.1° Sodium vaporization from Beulah lignite began at 1073 K and peaked at 1273 K, all sodium was (4) Huffman, G.P.; Huggins, F. E. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society: Washington, DC, 1986; Chapter 8. (5) Sondreal, E. A.; Gronhovd, G. H.; Severson, D. E. Alkali Metals in Low-Rank Coals-A Critical Review and Analysis of Research Datu on Their Occurrence and Effect in Combustion and Gasification Systems; Coal Energy Technology Consultants: Grand Forks, ND,
-.,"-.
1 QRF.
(6) Raask, E.; Williams, D. M. J . Inst. Fuel 1966, 38, 255. (7) Watt, J. D. The Physical and Chemical Behavior of Mineral Matter in Coal Under the Conditions Met in Combustion Plant; British
Coal Utilization Research Association: Leatherhead, U.K., 1969. (8) Raask, E. J . Inst. Energy 1984, 57, 231. (9) Neville, M.;Sarofim, A. Fuel 1986, 64, 384. (10) Srinivasachar, S.;Helble, J. J.; Ham, D. 0.;Domazetis, G . Prog.
Energy Combust. Sei. 1990, 16, 303.
0 1994 American Chemical Society
Combustion of Beulah Pulverized Coal
vaporized above 1923 K (ref 10). The rate of vaporization is determined by the reduction of oxides to a more volatile material in the locally reducing zone. Thus, atomic sodium is favored under reducing conditions in the flame, while sodium hydroxide is the dominant vapor species in oxidizing conditions. Sodium that is volatilized may react with flame-borne silicate particles of sulfur oxides under postcombustion oxidizing conditions.lOJ1 The direct incorporation of sodium in silicates forms new mixed silicate phases. These alkali silicates tend to have lower melting points and viscosities, encouraging formation of a melt phase at lower temperatures and subsequent sintering. Sulfation of alkali species accounts for the large percentage of sulfur retained in ash during combustion of low-rank coals. Sulfation occurs either homogeneously in the gas phase or heterogeneously on particle surfaces. Recent work has suggested that formation of sodium sulfate from the vapor phase is not kinetically favored and that condensation of sodium hydroxide beyond the flame zone and its subsequent reaction with gaseous sulfur dioxide is the favored mechanism for formation of sodium sulfate.1° Heterogeneous reactions result in smaller particles that are enriched in sodium due to condensation on the surface of smaller particles. A significant number of sulfate particles may form directly on the surface of ash particles. The mechanism involves the capture of volatilized alkalis and alkaline earths, as oxides, at the surface of silicate particles low in alkali content. The oxidized alkalis and alkaline earths are subsequently sulfated by the sulfur oxides in the gas stream. Potassium and sodium sulfates tend to be the dominant sulfate species concentrated a t the surface of silicate particles.l1 Vaporized potassium behaves similarly to sodium. However, potassium is often associated with mineral species, such as illite, in bituminous coals. The mineralbound potassium is less likely t o volatilize during combustion. For example, only 2-11% of the total potassium was vaporized from synthetic chars doped with illite.12 The amount of potassium in the submicrometer fume correlated with the amount of sodium, suggesting that mobile sodium may displace potassium from an aluminosilicate, thereby providing a means by which mineral-bound potassium may be volatilized.12 Organically bound calcium is an important fluxing agent in low-rank coals. Calcium oxide particulate matter grows rapidly from highly dispersed volatilized calcium species, thus removing from the vapor phase calcium species which might otherwise react with aluminosilicate parti~1es.l~ The incorporation of calcium into aluminosilicates will generally lower the melting point and viscosity of the melt phase.14 However, incorporation of large amounts of calcium may increase (11)Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: Washington, DC, 1985. (12) Helble, J. J.; Srinivasachar, S.; Boni, A. A,; Bool, L. E.; Gallagher, N. B.; Peterson, T. W.; Wendt, J. 0. L.; Huggins, F. E.; Shah, N.; H f f i a n , G. P.; Graham, K A.; Sarofim, A. F.; Beer, J. M. Inorganic Transformations and Ash Deposition During Combustion; Benson, S. A., Ed.; ASME: New York, 1992; pp 209-228. (13)Helble, J. J.; Srinivasachar, S.; Boni, A. A.; Kang, S. G.; Graham, K. A.; Sarofim, A. F.; Beer, J. M.; Gallagher, N. B.; Bool, L. E.; Peterson, T. W.; Wendt, J. 0. L.; Shah, N.; Huggins, F. E.; H f f i a n , G. P. Inorganic Transformations and Ash Deposition During Combustion; Benson, S. A., Ed.; ASME: New York, 1992; pp 229-248. (14)Jung, B. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990.
Energy & Fuels, Vol. 8, No. 6, 1994 1209 the values of these properties, possibly reducing the extent of sintering or agglomeration.15J6 At 120% excess oxygen, less than 5% of the calcium was vaporized from Beulah lignite and synthetic char loaded with calcium by ion exchange, but the amount of calcium volatilized increases with excess oxygen.13 Alkalis and alkaline earths not volatilized are incorporated in minerals, primarily silicates. Sulfation of the nonvolatilized alkalis and alkaline earth elements may occur at the surface of alkali-rich silicates. For example, potassium sulfate may form directly on the surface of potassium aluminosilicates (Le., clays) in the coal.ll Alkali-rich silicates that have submicrometer sulfate particles or coatings on their surfaces have been observed. These submicrometer particles or coatings are typically composed of calcium or sodium sulfate and are formed by capture of the nucleated or condensed oxidized alkalis and alkaline earths by silicates. These captured species are then sulfated. The particles or coatings are not formed by direct sulfation of alkali-rich silicates. Whether mineral particles are inherent or extraneous is significant for their time-temperature history (as well as affecting their proximity to other mineral particles, and the local oxidizing or reducing environment). Inherent mineral matter is that associated intimately with the carbonaceous portion of the fuel, whereas extraneous mineral matter is entirely free of the carbonaceous portion. An example relevant to the present discussion is gypsum. Gypsum is present primarily in low-rank coals. It dehydrates completely at 436 K, forming anhydrite, which begins to melt at 1725 K. A pulverized coal flame often reaches the melting point of anhydrite and may initiate the formatiion of a sticky layer on the surface of extraneous Cas04 particles. Since the residence time of the particle in the flame is very short, the particles rarely reach a complete melt phase. Once these particles exit the flame, they are rapidly quenched as gas temperatures decrease. If gypsum is present as finely dispersed inherent mineral matter in the coal, it may form a melt phase within the char and react there with other inorganics. The maximum temperature experienced by the inherent Cas04 is greater than that experienced by particles free within the gas stream. In addition, the duration that these Cas04 particles are maintained at elevated temperatures is longer than for extraneous Cas04 particles, due to the time required for char burnout. The additional time at elevated temperature allows for the formation of additional melt phases and reactions with other inorganics. Some calcium and magnesium are also incorporated in carbonates: calcite, ankerite, magnesite, and dolomite. Carbonates decompose on heating, forming the appropriate oxide. After decomposition of CaC03, the remaining CaO is very reactive with sulfur gases. The calcium sulfate formed in this process often appears as a coating on other ash particles, as well as an initial deposit layer on heat exchange surfaces which acts as a capture surface for subsequent deposits. CaO and MgO that are not sulfated are often incorporated into silicate or aluminosilicate phases. Inherent calcite often acts as a fluxing agent in silicates. (15) Falcone, S. K. U S . Dept. of Energy Rep. DE-FC21-83FE60181, 1986. (16) Jung, B.; Schobert, H. H. Energy Fuels 1991,5, 555.
Miller and Schobert
1210 Energy & Fuels, Vol. 8, No. 6, 1994 Several models have been proposed for the formation of ash during the combustion of pulverized c0al.11J7-24 Models of the combustion of coal-water slurry fuels (CWSF) and ash formation processes have begun to appear in the literature in recent However, most of the studies on the CWSF combustion process have not had the elucidation of inorganic transformations as their main focus. The gaseous environment, proximity of other mineral particles, and thermal history of mineral matter in a CWSF can potentially be quite different from those experienced by the mineral matter in the same coal burned in a conventional pulverized coal form. In a previous paper and in a companion paper, we have presented results on the behavior of iron compounds35and silicate and aluminosilicate minerals36 in comparative ash formation processes in pulverized coal and CWSF combustion. In the present paper we extend our previous work t o discuss aspects of the behavior of selected alkali and alkaline earth elements and sulfur.
Experimental Section
Table 1. Residence Time and Burnout of Test Fuels and Char Ash at Different Locations within the Furnace pulverized coal port 1 port 2 port 10 coal-water slurry fuel port 3 port 4 port 8 port 10
residence time (s) 0.16 0.41 2.09 0.25 0.41
1.03 1.68
burnout" 10.8 83.1 99.6
-
70.9 76.6 97.6 97.8
Burnout calculations based on ash tracer technique. Ash percents on an oxidized basis obtained from the thermogravimetric analyzer. (m3.h). Sampling ports along t h e combustor are numbered sequentially from 1 (top) to 10 (at the bottom). Residence times are given in Table 1. Particles were sampled isokinetically and classified using a water-cooled sampling probe and a three-stage Anderson multicyclone and filter assembly. The cyclone aerodynamic diameter 50% cutpoints (Le., 50% of the particles are less t h a n the indicated size) were 15 pm (cyclone 11, 2.54 pm (cyclone 21, and 0.42 pm (cyclone 3) for the pulverized coal tests and 15 pm (cyclone 11, 3.22 pm (cyclone 21, and 0.57 pm (cyclone 3) for the coal-water fuel tests. Maximum gas temperature measured during combustion of the pulverized coal was 1800 and 1688 K for the CWSF. Detailed combustion temperature and gas profiles are published e l s e ~ h e r e . ~ Full ~.~ details ~ of the design and operation of the combustor and ancillary sampling probes are given
Fuel Preparation and Characteristics. The Beulah lignite (Beulah-Zap seam, Mercer County, North Dakota) h a s an average ash content of 8.6 and 0.9% sulfur. The CWSF was prepared a t t h e University of North Dakota Energy and Environmental Research Center by hydrothermal treatment.39 The CWSF had a solids loading of 50%. Details of preparation and characterization of the fuels are presented e l s e ~ h e r e . ~ ~ ~ ~ ~ Analytical Techniques. Analyses of ash samples were Combustion Facility. The fuels were fired at 316 M J h , performed using a Spectrometrics Spectroscan 3 direct current corresponding t o a volumetric heat release rate of 1.58 GJ/ plasma (DCP) spectrometer following lithium metaborate fusion. Elements are reported on a n oxide basis and normal(17)Sarofim, A. F.; Howard, J. B.; Padia, A. S. Combust. Sci. ized to 100%. so3 is reported as the original normalized value; Technol. 1977,16,187. all other elements are reported on a n SO3-free basis and (18)Bryers, R. W. Proceedings of the Symposium on Slagging and Fouling in Steam Generators; Brigham Young University: Provo, UT, renormalized t o 100%. 1987. The portions of elements present as cations or mineral (19) Helble, J. J.; Srinivasachar, S.; Katz, C. B.; Boni, A. A. Prepr. phases was determined by fractionation using extraction with Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,34,383. (20) Gallagher, N. B.; Peterson, T. W.; Wendt, J. 0. L. Prepr. 1M NH40Ac (three extractions at 70 "C for 24 h), followed by Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36,181. extraction of the residue with 1 M HC1 at the same conditions. (21)Helble, J. J.; Neville, M.; Sarofim, A. F. Proc. Symp. (Int.1 DCP analysis of the filtrate after NH40Ac extraction identifies Combust. [Proc.]21 1986,411. that portion of a n element present as ion-exchangeable cations; (22) Kang, S. G.; Charon, 0.; Sarofim, A. F.; Beer, J . M. Proc. 6th Int. Pittsburgh Coal Conf. 1989,74. analysis of the HCl extract identifies the portion present a s (23) Wilemski, G.; Srinivasachar, S.; Sarofim, A. F. Inorganic acid-soluble minerals or coordination complexes. Species Transformations and Ash Deposition During Combustion; Benson, S . remaining in the residue after HC1 extraction are mostly acidA., Ed.; ASME: New York, 1992; pp 545-564. insoluble minerals such as aluminosilicates, quartz, and pyrite. (24) Zygarlicke, C. J.; Ramanathan, M.; Erickson, T. A. Inorganic Transformations and Ash Deposition During Combustion; Benson, S . Computer-controlled scanning electron microscopy (CCSEM) A,, Ed.; ASME: New York, 1992, pp 525-544. was used to identify and quantify the size and composition of (25) Monroe, L. S.; Farmayan, W. F.; Srinivasachar, S.; Beer, J. M. discrete inorganic phases in the original coals and in ash In Slagging and Fouling Due to Impurities in Combustion Gases; Barrett, R. E., Ed.; United Engineering Trustees: New York, 1989; samples collected from the combustor. Procedures have been pp 683-712. published elsewhere.42 Particles are classified by the com(26) Kang, S. W.; Sarofim, A. F.; Teare, J. D.; Beer, J. M. MIT Energy puter, on the basis of composition, to one of 33 specific mineral Laboratory Rep. MIT-EL-87-002, 1987. phases or assemblages. The total area of the particles as (27) Dunn-Rankin, D.; Hoornstra, J.; Holve, D. J. Sandia National Laboratory Rep. SAND 86-8769, 1986. measured by CCSEM is used, in conjunction with known or (28) Matthews, K. J.; Jones, A. R. Proc. 8th Int. Symp. Coal Slurry calculated densities, to calculate the weight percent of each Fuels Prepn. Utiliz. 1986,388. mineral phase in t h e different size categories. The analyses (29) Beer, J . M. Proc. Second Eur. Conf. Coal Liq. Mixtures 1985, were conducted at UNDEERC. The average number of 277 _ .. . (30) Kang, S. W.; Sarofim, A. F.; Beer, J. M. Proc. Third Eur. Conf. particles classified in the samples was 1675, with a range of Coal Liq. Mixtures 1987,179. 1054-2278. The standard deviation, at a 68% confidence (31) Zhou, Z. Q.; HamdullahDur. F. Proc. 3rd Eur. Conf. Coal Lio. limit, of the data obtained by CCSEM is expressed by eqs 1 Mixtures 1987,151. and 2 for mineral quantities and for particle size distribution, (32) Murdoch, P. L.; Pourkashanian, M.; Williams, A. Symp. (Int.) Combust. [Proc.]20 1984,1409. respectively, where N represents the number of particles (33) Holve, D. J.; Gomi, K.; Fletcher, T. H. Sandia National Laboratory Rep. SAND 85-8706, 1985. (34) Boni, A. A.; Garman, A. R.; Johnson, S. A.; Thames, J. M. Proc. (39) Potas, T. A.; Baker, G. G.; Maas, D. J. J. Coal Qual. 1987,6 , 6th Int. Conf. Coal Water Slurry Combust. Technol. 1984,667. 53. (35) Miller, S. Falcone; Schobert, H. H. Energy Fuels 1993,7,1030. (40)Ramachandran, P. Ph.D. Dissertation, The Pennsylvania State (36) Miller, S. Falcone; Schobert, H. H. Energy Fuels, preceding University, University Park, PA, 1990. Daner in this issue. (41) Hurley, J. P. Ph.D. Dissertation, The Pennsylvania State (37) Miller, S. F. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990. University, University Park, PA, 1992. (42) Zygarlicke, C. J.; Steadman, E. N. Scanning Microsc. 1990,4, (38) Miller, S. Falcone; Schobert, H. H. Energy Fuels 1993,7,520. 579. ~~
~
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Energy & Fuels, Vol. 8, No. 6, 1994 1211
Combustion of Beulah Pulverized Coal ~
Table 2. DCP Chemical Analysis (wt %) of the Beulah Pulverized Coal and Coal-Water Slurry Fuel Chars and Ashesa Si02 A1203 Ti02 Fez03 MgO CaO MnO NazO KzO SO3 ~~
~
pulverized coal lab ashb port 1 port 2 port 10 coal-water slurry fuel lab ash port 3 port 4 port 8 port 10
23.9 22.2 26.6 26.1
13.2 14.2 14.6 13.3
0.6 0.8 0.6 0.8
13.2 12.7 12.1 12.7
8.0 8.8 8.8 8.1
27.7 28.1 28.4 28.9
0.1 0.1 0.1 0.1
12.7 12.6 8.5 9.9
0.6 0.7 0.3 0.3
21.5 21.4 9.9 6.1
26.7 29.6 26.0 24.5 22.3
12.5 12.0 12.3 13.5 13.4
0.6 0.7 0.7 0.7 0.6
15.3 16.6 17.4 12.9 14.2
8.0 7.7 8.3 9.2 9.6
27.6 28.1 30.4 31.5 33.3
0.1 0.1 0.1 0.1 0.1
8.5 4.7 4.5 7.3 6.3
0.6 0.5 0.3 0.5 0.3
17.6 12.2 11.8 5.8 5.4
a All elemental oxide weight percents on a the MAC-400.
so3 free basis except for sos.
Table 3. Beulah Pulverized Coal Chemical FractionationData initial removed by (uglg of HzO and dry coal) NH40Ac (%)“ 6 silicon 7204 4516 0 aluminum 20 titanium 247 11 5975 iron 95 magnesium 3111 79 calcium 12717 50 manganese 46 99 sodium 6072 46 potassium 306 nad phosphorus 251 77 sulfur 7062 1075 44 barium na strontium 580
Lab ash refers to the laboratory prepared ash produced in
Table 4. Beulah Coal-Water Slurry Fuel Chemical Fractionation Data
removed by remaining HCl (%Ib (%Ic 94 0 49 51 3 77 63 26 3 2 20 1 47 3 0 1 6 48 na na 23 0 1 55 na na
*
silicon aluminum titanium iron magnesium calcium manganese sodium potassium phosphorus sulfur barium strontium
initial removed by (uglg of H2O and removed by remaining dry coal) NH40Ac (%la HC1(%) 86’0)” 17 1 82 8792 4660 3 30 67 266 19 0 81 7522 19 35 46 3394 84 13 3 13807 70 29 1 35 57 8 55 4436 97 0 3 369 46 21 33 262 nad na na 6003 63 35 2 38 17 45 1104 604 na na na
a Water-soluble and ion-exchangable. Acid-soluble mineral matter. Acid-insoluble mineral matter. na, not available. Insufficient sample for analysis.
a Water-soluble and ion-exchangeable. Acid-soluble mineral matter. Acid-insoluble mineral matter. na, not available. Insufficient sample for analysis.
analyzed for a particular size range.43
Results and Discussion Characterization of Fuels. The alkali species, expressed as equivalent oxides, account for 49 and 44.7% (by weight on an SO3-free basis) of the total inorganic species in Beulah pulverized coal and CWSF, respectively (Table 2). In addition, 86.3 and 77.2% of these elements are present as organically bound inorganics in the coal and CWSF respectively (Tables 3 and 4). The organically bound alkalis and alkaline earth elements are much more reactive and abundant than mineral-bound alkalis and alkaline earths. In a companion paper, we have shown that these elements are very important in reacting with silicates and aluminosilicates to form new phases in the Beulah ashes.36 These new inorganic phases generally have a greater propensity to coalesce and agglomerate during combustion due to their lower melting points, viscosities, and surface tension^.^^,^^,^^ Such new phases are also important in the forming of ash deposits in a combustor, and in both the rate of formation of, and growth of strength in, the deposit^.^^^^^
CCSEM analysis of the inorganic phases identified in the two fuels and their respective ash is given in Table 5 . The total sulfur and sulfur forms identified in the Beulah fuels are given in Table 6. In low-rank coals, a larger portion of the sulfur in the coal is retained in the ash, as compared to higher rank coals. On a wholecoal basis, the retention of sulfur in the Beulah pulverized coal and CWSF ashes in 24.4 and 24.0%, respectively. Sulfur retention was calculated on a whole-coal basis rather than on the basis of laboratory-prepared ash since retention of sulfur in low-rank coal ashes prepared in the laboratory is not always an accurate measure of sulfur levels to be expected in ash formed in an actual combustor. This discrepancy is due to the lower rank coals containing a greater percentage of organically bound inorganics which volatilize during combustion and react in the gas stream. Sulfur that volatilizes during combustion often exits the combustor as a component of the combustion gas. Under laboratory conditions, the sulfur may be fixed in the ash due to reactions with organically bound cations at slower heating rates. Sulfur is present in the Beulah fuels primarily (Le., 62%) as organic sulfur (Table 6). In both fuels, so3 present in the port 10 ashes only represents 28 and 31% of the amount of SO3 measured in the ashes prepared
(43) Zygarlicke, C. J.; Erickson, T. A.; Murali Ramanathan; Toman, D. L. Presented at the Seventh Annual Coal Preparation, Utilization and Environmental Control Contractors Conference, July 1991. (44) Vorres, K. S.; Greenberg, S.; Poeppel, R. B. Argonne National Laboratory Rep. ANJJFE-85-10, 1985. (45) Falcone, S. K. U.S. Dept. ofEnergy Rep. DE-FC21-85MC10637, 1987.
(46) Gronhovd, G. H.; Beckering, W.; Tufte, P. H. ASME Paper 69WMCD-I, 1969. (47) Rindt, D. K.; Selle, S. J.; Beckering, W. ASME Paper 79-WN CD-5, 1979. (48) Benson, S. A. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1986.
SD = (2.2/N)lf2 SD = (3.0/N)”2
(2)
1212 Energy & Fuels, Vol. 8, No. 6, 1994
Miller and Schobert
Table 5. Inorganic Phases Identified In the Beulah Pulverized Coal and Coal-Water Slurry Fuels and Ashes
m
volume % of total inorganics mineral Dhase
PC
quartz iron oxide dolomite kaolinite montmorillonite K-AI silicate Ca-Al silicate Na-Al silicate aluminosilicate mixed Al silicates Ca silicates Ca aluminate pyrite pyrrhotite oxidized pyrrhotite gypsum barite gypsudAl silicate Ca-Al-P Si-rich Ca-rich Ca-Si rich traceb unknown
12.2 1.6 6.4 2.3 3.4 1.4 1.3 1.3 25.5 3.8 2.2 9.4 3.1 2.8 7.1 4.6 11.6
total
100.0
-a
PCash
CWSF
CWSFash
13.7 2.4 2.8 -
32.9 8.5 1.3 1.0 -
13.6 2.7 1.8 -
-
2.6 3.1 -
-
-
4.1 2.6
24.7 -
-
-
-
-
2.8
-
4.5 6.4 5.8 49.2 100.0
2.8 2.7 1.3 -
-
2.6 4.9 2.8
26
L
1 s
\
~
I 1
!
Fuel Ash 1.1
1.2
1-3
2.2 S.mplc
2-1
2-3
10.1
10.2
10.3
[PO". Cyc1"
Figure 1. Weight percent of calcium oxide in the Beulah
1.2 1.1 2.5
pulverized coal laboratory-prepared ash and cyclone char samples.
-
3.4 21.4 100.0
100.0
Minerals or inorganic assemblages comprising less than 1%, by volume, of the total inorganic material. Includes minerals or inorganic assemblages making up less than l%, by area, of the total inorganic material. In addition to the phases listed in this table trace amounts of the following minerals were identified: rutile, alumina, calcite, Fe-AI silicates, and gypsumharite. a
Table 6. Total Sulfur and Forms of Sulfur Identified in Test Fuels total sulfup sulfur forms' sulfate sulfur pyritic sulfur organic sulfur
27.1
-
2.2 7.4 2.4 3.0 51.8
-
5
PC"
CWSFb
1.09
1.04
0.04 0.39 0.66
0.26 0.14 0.64
a Pulverized coal. Coal-water slurry fuel. Weight percent on a whole coal and dry basis.
from the pulverized coal and CWSF in the laboratory. The concentration of SO3 in the char samples collected at each port decreases with burnout and residence time (Table 2). The greatest decrease in SO3 content of the chars occurs between ports 1 and 2 for the pulverized coal and ports 4 and 8 for the CWSF. This corresponds to those ports where the greatest increase in burnout and maximum gas temperatures occur. Calcium is the most abundant inorganic element in these fuels. CaO in the laboratory-prepared ash accounts for 27.7 and 27.6% of the total oxides (Table 2). Seventy-nine and 70% of the calcium in the pulverized coal and CWSF, respectively, are present in an organically bound form (Tables 3 and 4). In a companion paper, we have shown that, during combustion, the majority of calcium reacts with aluminosilicates within the char.36 Very little organically bound calcium escapes the char particle into the gas stream.13 The calcium that does escape may react with SO3 in the combustion gas, or, to a minor extent, may also react at the surface of silicate particles in the gas stream. The majority of the calcium in the two Beulah fuels is retained in their respective ashes. The only mineral phases in the Beulah fuels that contain significant calcium are calcite and gypsum
(Table 5). Both form CaO on decomposition. However, mineral-bound calcium becomes available at higher temperatures than organically bound calcium. If calcite and gypsum are finely dispersed within the coal particle, the CaO may react with inherent silicates and aluminosilicates. However, larger extraneous calcite particles rapidly decompose in the oxidizing gas stream and may subsequently react with SO3 in the gas. The decomposition of calcite and subsequent reaction with sulfur during combustion occurs beyond the flame zone where temperatures are 11725 K. Gypsum rapidly forms Cas04 (anhydrite) in heating. These calcium sulfate particles may be partially molten in the flame. The CCSEM does not consider classification of calcium sulfates in the Beulah ashes. These particles are identified as "unknown" within the CCSEM classification scheme. However, the DCP data do allow some interpretation of calcium reactions during combustion. For combustion of the pulverized coal and the CWSF, a majority of the CaO is retained in the final ash (the slight increase reported at port 10 is an artifact of the data, associated with a decrease of so3 and NazO, Table 2). The distribution of CaO in the cyclone samples collected during combustion of the pulverized coal is shown in Figure 1. At port 1,the concentration of CaO shows no partitioning among the cyclones, suggesting that a majority of the organically bound calcium is still dispersed evenly in the char, or that CaO has reacted equally with all particle sizes. At ports 2 and 10, the concentration of CaO in cyclone 2 is slightly greater than in cyclones 1or 3. Cyclone 2 had a d50 cutpoint of 2.54 pm. This partitioning is related to reaction of calcium with aluminosilicates and silicates within the char particle, and subsequent coalescence and agglomeration. Results presented in a companion paper show the concentration of calcium silicate phases increases in the larger ash particles with increased burnCCSEM analysis shows an increase in the volume percent of calcium silicates, calcium aluminosilicates, and calcium-rich phases in the pulverized coal and CWSF ash. Continued coalescence and agglomeration with increased burnout results in a decrease in CaO concentration in cyclone 3 from port 2 to port 10 (Figure 1). The similar distribution pattern at ports 2 and 10 suggests that most of the partitioning of CaO is complete by port 2, where burnout is 83.1%. In the cae of the CWSF, the distribution pattern of CaO is well established at port 3 (Figure 2). The burnout at port 3 is 70.9% and the gas temperature is
Combustion of Beulah Pulverized Coal
Energy &Fuels, Vol. 8, No. 6, 1994 1213
26 Fuel A s h 3 . I
3.2
3-3
4.1
4.2
4.3
8-1
8.2
8.3
10-1
10.2
10.3
Sample [Po" - cy.ionc1
Figure 2. Weight percent of calcium oxide in the Beulah coal-water slurry fuel laboratory-prepared ash and cyclone char samples.
"
,
Furl Ash
1-1
1.2
2-1 2.2 Sslnple
1.3
2.3
10.1
10.2
10-3
lPorr . Cycloncl
Figure 3. Weight percent of calcium oxide and sulfur trioxide in the Beulah pulverized coal laboratory-prepared a s h and cyclone char samples. I
40,
P 0
1 ,
* ,
Fuel A i h 3 . 1
,
,
3.2
3.3
,
,
4.1
4.2
,
,
,
,
,
,
4.3
8.1
8.2
8.3
10-1
10.2
, I
10-3
S"ll lPori. Cycloncl
Figure 4. Weight percent of calcium oxide and sulfur trioxide in the Beulah coal-water slurry fuel laboratory prepared ash and cyclone char samples.
1708 K. The concentrating of CaO in cyclone 2 is again associated with aluminosilicate formation. With continued burnout, coalescence and agglomeration of aluminosilicate particles increase as the char surface recedes. As the aluminosilicate particle size increases, the concentration in the smaller cyclone size fractions decreases. The fact that the concentration of CaO does not shift to smaller size fractions suggests that calcium is not involved in significant nucleation or condensation reactions that would result in formation of submicrometer-size particles. The concentration of CaO diminishes with particle size in both fuels. However, the concentration of SO3 increases with decreasing particle size (Figures 3 and 4). This suggests there is not a direct association of calcium and sulfur as would be expected
if calcium species in the gas were reacting primarily with gaseous sulfur compounds, forming calcium sulfate in the gas phase. Virtually all of the sodium is associated with the organic portion of the coal (=97% is organically bound). Sodium is present in lower amounts in the CWSF than in the pulverized coal (Tables 3 and 4); this is attributed to sodium removal by the process used to prepare the CWSF. Sodium reactions at the char surface and within the gas stream are possible.20 Heterogeneous condensation of NaOH and subsequent sulfation are most likely responsible for formation of sodium sulfate.1° Alkali and alkaline earth species may condense when the combustion products cool or when chemical reactions produce less volatile species.49 Whether condensation or nucleation of sodium species occurs depends on the cooling rate, the chemical composition of the vapors, and the size distribution of the existing aerosol.49 If sodium species are preferentially reacting with sulfur oxides during combustion,forming very small NazS04 particles by nucleation, there should be a direct relation between the concentrations of sodium and sulfur in the char samples. Also, the concentration of sodium would be expected to be greatest in the smaller size fractions (Le., cyclone 3). On the other hand, if condensation of sodium species on the surfaces of larger ash particles (e.g., aluminosilicates) and subsequent sulfation is the main mechanism for Na~S04formation, then NazO should be distributed over a greater size range, and the relation of sodium and sulfur concentrations may be less obvious. Approximately 78 and 74%of the NazO in the original fuels is retained in the pulverized coal and CWSF ashes, respectively (Table 2). The concentration of NazO decreases from port 1 to port 2, and subsequently increases at port 10. Substantial amounts of sodium can be vaporized under the reducing conditions occurring at the surface of a burning coal particle.49 Some release of sodium is occurring at port 1. At port 2, the NazO concentration is at a minimum and the gas temperature is at a maximum.37 This is the point at which the greatest amount of volatilized sodium is present in the system. Once the combustion gases begin to cool past port 2, the sodium vapor begins to react with other species in the gas stream. The result is the increase in NazO at port 10. For the CWSF, NazO is low at ports 3 and 4 relative to the laboratory-prepared ash (Table 2). Sodium vaporization has begun by port 3. Maximum gas temperatures during combustion of the CWSF occur at ports 3 and 4 (refs 37 and 38). The concentration of NazO then increases at port 8. The distributions of NazO (SOa-free basis) and SO3 for the pulverized coal and CWSF char samples collected down the combustor are shown in Figures 5 and 6 , respectively. The distribution of NazO among the cyclones is quite different than the distribution of CaO. In both cases, the concentration of NazO is greatest in cyclone 3, and the partitioning of NazO among different particle sizes increases down the combustor. With pulverized coal, partitioning of sodium is apparent by port 2, consistent with sodium vaporization occurring between ports 1 and 2. Also, enrichment of sodium in the third cyclone has already begun, due to formation of submicrometer particles by nucleation. With the (49) Ulrich, G. D.; Riehl, J. W.; French, B. R.; Desrosiers, R. ASME Int. Symp. Corrosion Deposits Combust. Gases 1977.
Miller and Schobert
1214 Energy & Fuels, Vol. 8, No. 6,1994 ”7
-7
.
I
2.0)
1.0
d 0-
I
he1 Ash
1-1
1-2
1-3
2-1 2-2 Sample
2-3
10.2
10.1
10-3
-
IM C F h l
Figure 5. Weight percent of sodium oxide and sulfur trioxide in the Beulah pulverized coal laboratory-prepared ash and cyclone char samples.
0
’
.
I
Fuel Ash3.l
I
3-2
.
3-3
1
4-1
.
I
4-3
4-2
I
1
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8-2
.
8-3
B 1
I
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10-2
i
o.5m 0.0
7
Fuel Ash 1-1
1-2
1-3
I
I
10-1
10-2
- cyelw)
the Beulah pulverized coal laboratory-prepared ash and cyclone char samples.
15
-
1.0
-
SAMPLE
Figure 6. Weight percent of sodium oxide and sulfur trioxide
2-3
Figure 7. Molar ratio of sodium oxide to sulfur trioxide in
. ’
-
2-2
Sample
(pori
10.3
lpm C F h l
2-1
Fuel A i h 3 - l
3-2
3-3
4-1
I
4-2
4-3
ti-1
8-2
8-3
10-1
10-2
10-3
Smmplc
-
(pan C F h )
in the Beulah coal-water slurry fuel laboratory-preparedash and cyclone char samples.
Figure 8. Molar ratio of sodium oxide to sulfur trioxide in
CWSF, partitioning of sodium among cyclones has already occurred at port 3, indicating that much of the organically bound sodium has been released from the coal particle by port 3. The SO3 concentrations are also greatest in the cyclone 3 samples (Figures 5 and 6). The fluctuations in SO3 concentration mirror those of Na20, suggesting reaction during combustion to form Na2SO4. In the case of the CWSF, partitioning of Na2O among the different cyclones is seen a t port 3, but partitioning of SO3 id not apparent until port 4. This may reflect the extent of burnout and the release of organic sulfur in the smaller char particles relative to the larger ones. The Na20/S03 molar ratios for cyclone samples collected at different ports are shown in Figures 7 and 8. The Na20/S03 molar ratio for Na2S04 is 1. Sodium is probably present as Na2S04 in those cyclones where the Na20/S03 molar ratios are -1. The submicrometer particles collected in cyclone 3 typically had Na20/S03 molar ratios of =l. These represent particles formed by the preferential reaction of volatilized sodium and sulfur oxides during combustion. Samples having molar ratios ’1 indicate incorporation of sodium into larger aluminosilicate particles. Sodium sulfate was observed as coatings on larger particles collected in cyclone 1.The increase in the Na20/S03 molar ratio in the CWSF char collected in cyclones 1and 2 a t port 10 is a reflection of the greater extent of coalescence and agglomeration of aluminosilicatesincorporating sodium within the CWSF ash; a greater range of sizes of the CWSF ash particles experiences coalescence during combustion as compared to the pulverized coal ash.
Particles having significant sulfur content are classified as “unknown” in the CCSEM analysis. Analysis of the particles classified as “unknown” by CCSEM analysis showed a significant number of particles containing sulfur associated with sodium. Sodium aluminosilicate, not originally present in the coal, was identified in the pulverized coal ash suggesting that sodium reacted with other aluminosilicates during burnout (Table 5). However, no sodium aluminosilicates were identified in the CWSF ash. Examination of the raw CCSEM data shows that a significant percentage of the “unknown” particles identified in the CWSF ash contain sodium, sulfur, aluminum, and silicon but not in proportions to be classified as sodium aluminosilicate. K20 accounts for only 0.6% of the total inorganics in the laboratory-prepared ashes of the Beulah fuels. Forty-six percent of the potassium is present as an organically bound cation; the remainder is associated with clay minerals (Tables 3 and 4). Potassium aluminosilicates account for 2.5 and 0.4% of the total mineral matter in the pulverized coal and CWSF, respectively (Table 5). However, no particles classified as potassium aluminosilicates are in the ash samples collected at port 10. Potassium minerals vitrify in the flame; a portion of the potassium is retained in a glassy phase while another portion may escape into the gas.ll Once in a vapor phase, the potassium species react with SO3 and form K2S04 and may also react a t the surface of silicates and aluminosilicates, similarly to CaO and Na2O. The concentration of K20 in the pulverized coal char is greatest in the smaller size fractions. The partition-
the Beulah coal-water slurry fuel laboratory-prepared ash and cyclone char samples.
Energy &Fuels, Vol. 8, No. 6,1994 1215
Combustion of Beulah Pulverized Coal 11.0 1
10.5 10.0
2
E
2 %
.I
M
3
Fuel Ash
1-1
1.2
1-3
2.1
2.2
2.3
10.1
10.2
10.3
S.mplr (Po" .Cycloncl
Figure 9. Weight percent of potassium oxide in the Beulah pulverized coal laboratory-prepared ash and cyclone char
samples. ing of K20 between cyclones 1and 3 increases from port 1to port 10 (Figure 9). The difference in concentrations of K20 between cyclones 2 and 3 is greatest at port 2. The pattern of K20 concentration in the pulverized coal cyclone samples is similar to that of SO3. There is an enrichment in the larger particle sizes either due to condensation of K2S04 on particle surfaces or formation of larger particles by coalescence. The increased concentration of K20 in cyclone 3 is due to formation of submicron K2SO4 particles. The distribution of K20 in the CWSF cyclone char samples also shows an increase in K20 in the smaller particle sizes. Partitioning is apparent in the port 3 chars, suggesting that volatilization of potassium has occurred by this time in the combustion process. With increased burnout and residence time, the concentration of K20 in the smaller size particles increases. Increased K20 in cyclone 3 is due to the formation of submicron K2S04 particles; increased K20 in the second cyclone may be due to the formation of surface coatings on larger particles. No particles classified as potassium aluminosilicates were collected at port 10 for either fuel (Table 5). The data suggest that the amount of potassium present in the potassium aluminosilicates is reduced during combustion such that the total percent of X-ray counts for K, Al, and Si is
